1. Field of the Invention
The present invention relates to a driving method for an ink ejection device. More particularly, the invention relates to a driving method for an ink ejection device capable of ejecting ink droplets regardless of change in environmental temperature.
2. Description of Related Art
Non-impact type printing devices have recently taken the place of conventional impact type printing devices and are holding an ever-growing share of the market. Of these non-impact type printing devices, ink-ejecting type printing devices have the simplest operation principle, but are still capable of effectively and easily performing multi-gradation and color printing. Of these devices, a drop-on-demand type for ejecting only ink droplets required used for printing has rapidly gained popularity because of its excellent ejection efficiency and low running cost.
Two representative drop-on-demand printers types are the Kyser type, such as disclosed in U.S. Pat. No. 3,946,398, and the thermal ejecting type, such as disclosed in U.S. Pat. No. 4,723,129. These types of drop-on-demand printers have disadvantages. The Kyser type is difficult to design in a compact size. The thermal ejecting type requires heat-resistant ink because the ink is heated to a high temperature when ejected.
A shear mode type printer, such as disclosed in U.S. Pat. No. 4,879,568, has been proposed to simultaneously solve the above disadvantages.
As shown schematically in FIGS. 1(a) and 1(b), the shear mode type of ink ejection device 600 comprises a bottom wall 601, a ceiling wall 602, and elongated shear mode actuator walls 603 sandwiched therebetween. Each actuator wall 603 includes a lower wall 607 adhesively attached to the bottom wall 601 and an upper wall 605 adhesively attached to the ceiling wall 602. The upper and lower walls 605, 607 are polarized in the directions indicated by arrows 609, 611 respectively. Alternating pairs of actuator walls 603 forms in alternation ink channels 613 therebetween or spaces 615, which are narrower than the ink channels 613.
A nozzle plate 617 is fixedly secured to one end of the actuator walls 603. The nozzle plate 617 is formed with nozzles 618 at positions corresponding to the ink channels 613. Electrodes 619 and 621 are provided on both side surface of each actuator wall 603. Each of the electrodes 619, 621 is covered by an insulating layer (not shown) to insulate it from the ink. The electrodes 621 confront spaces 615 and are connected to a ground 623. The electrodes 619 are provided in the ink channels 613 and are connected to an actuator driving circuit 625, such as a silicon chip. A manifold member 628 having a sealing plate 627 is fixedly secured to the other ends of the actuator walls 603. The ink channels 613 are in fluid communication with a common ink chamber 626 defined by the manifold member 628. The sealing plate 627 prevents ink from the common ink chamber 626 from entering the spaces 615.
Next, a method for manufacturing the ink ejection device 600 will be described. First, a piezoelectric ceramic layer polarized in the direction 611 is adhered to the bottom wall 601, and a piezoelectric ceramic layer polarized in a direction 609 is adhered to the ceiling wall 602. Incidentally the piezoelectric ceramic layers are formed to thicknesses required to produce the proper heights of the lower walls 607 and the upper walls 605. Next, parallel grooves are cut into the piezoelectric ceramic layers using a diamond cutting disc to form the lower walls 607 and the upper walls 605. The electrodes 619 and 621 are then formed on the side surfaces of the lower walls 607 using vacuum-deposition techniques. The insulating layer is formed onto the electrodes 619 and 621. Likewise, the electrodes 619 and 621 are provided on the side surfaces of the upper walls 605 and the electrodes 619 and 621 are covered by the insulating layer.
The vertices of the upper walls 605 and the lower walls 607 are adhered to one another to form the actuator walls 603, and consequently the ink channels 613 and the spaces 615. Next, the nozzle plate 617 formed with the nozzles 618 at positions corresponding to the ink channels 613 is attached to the ends of the actuator walls 603. The manifold member 628 is adhered to the other ends of actuator walls 603 so that the sealing plate 627 seals the spaces 615 to prevent ink from entering the spaces 615. The circuit 625 and the ground 623 are connected also to the other end of the actuator walls 603.
To eject droplets, a voltage from the actuator driving circuit 625 is applied to the electrodes 619, 921 of each ink channel 613. Pairs of the actuator walls 603 deform outward by the piezoelectric shear effect so that the volume of each ink channel 613 increases. In the example shown in FIG. 2, when a voltage V is applied to the electrodes 621c of the ink channel 613c, an electric field is generated in the actuator wall 603e in the direction indicated by arrows 631, 629 and an electric field is generated in the actuator wall 603f in the directions indicated by arrows 632, 630. Because the electric field directions 629 to 632 are at right angles to the polarization direction 609, 611, the actuator walls 603e, 603f deform outward to increase the volume of the ink channel 613c by the piezoelectric shear effect, resulting in a decrease in the pressure in the ink chamber 613c, including near the nozzle 618c. The negative pressure is maintained for a duration of time T corresponding to a duration of time required for a pressure wave to propagate once across the length of the ink channel 613c. During the time duration T, ink is supplied from the manifold 626.
The duration of time T can be calculated by the following formula: EQU T=L/a
wherein L is the length of the ink channel 613; and
a is the speed of sound through the ink filling channel 613.
Theories on pressure wave propagation teach that at the moment the duration of time L/a elapses after the rising edge of voltage, the pressure in the ink channel 613 inverts to a positive pressure. The voltage applied to the electrode 619c of the ink channel 613c is returned to 0V in synchronization with this inversion so that the actuator walls 603e, 603f revert to their initial shape shown in FIG. 1.
The pressure generated when the actuator walls 603e, 603f return to their initial shape is added to the inverted positive pressure so that a relatively high pressure is generated in the ink channel 613c. This relatively high pressure ejects an ink droplet 26 from the nozzle 618c.
As shown in FIG. 3, the viscosity of ink used in this type of ink ejection device varies greatly depending on the atmospheric temperature. In the example shown in FIG. 3, the ink has a viscosity of about 6 mPa.s at 25.degree. C. and about 6 mPa.s at 10.degree. C. Because the viscosity of ink varies with temperature in this manner, the speed of the ink droplet is slower at low temperatures than at high temperatures. Therefore, the printing position of ejected droplets on the printing medium cannot be precisely controlled, resulting in poor quality of printed images.
In an attempt to minimize variation in the speed of ejected ink droplets caused by changes in atmospheric temperature, the driving voltage of the ink ejection device has been changed in accordance with changes in atmospheric temperature. However, this driving method requires a complicated driving circuit because different voltage levels must be generated using a combination of two or more power sources. The configuration of the driving circuitry is thus costly.